Nitride semiconductors represented by gallium nitride (GaN) are semiconductors having an extremely broad band gap, and GaN and AlN have band gaps as broad as 3.4 eV and 6.2 eV, respectively. Further, GaN has characteristics that a breakdown field and a saturated drift velocity of electrons are two or three times greater than those of the other semiconductors, such as GaAs and Si.
Also, nitride semiconductors form multi-element mixed crystal semiconductors with the use of aluminum (Al) and indium (In) and a heterostructure can be designed by laminating semiconductors having different band gaps. For example, it is known that an extremely high sheet carrier concentration of 1.0×1013 cm−2 or higher can be obtained in a C-axis direction from spontaneous polarization and piezoelectric polarization generated by deformation resulting from lattice mismatch on a hetero interface of aluminum gallium nitride and gallium nitride having an Al composition ratio of 25%. This HEMT of AlGaN/GaN using a highly-concentrated two dimensional electron gas (2DEG) shows an extremely large and high driving capability, which is ten times greater than that of Si-based devices and about four times greater than that of a 2DEG based on AlGaAs/GaAs of the same compound semiconductor. Further, owing to the high capability of the material, nitride semiconductors realize on-resistance as low as or lower than 1/10 that of a MOSFET (Metal-Oxide Semiconductor Field-Effect Transistor) using Si and ⅓ that of an IGBT (Insulated Gate Bipolar Transistor) in a device having an on-resistance and a withstand voltage of 200 V as an element limit (for example, see NPL 1).
However, when the GaN-HEMT is applied to a power supply with an inductive load, an inverter having an inductive load motor, and the like, there are problems as follows.
In a case where an inductive load is connected to the HEMT, it is necessary to consume energy accumulated in the inductive load within the circuit when the HEMT is turned OFF. Herein, energy is expressed as: E=(½)×LI2, where L is self-inductance and I is a current. A MOSFET using Si has an anti-parallel parasitic diode connected between the drain and the source in a device structure. The cathode of the parasitic diode is connected to the drain and the anode is connected to the source. When the MOSFET is turned OFF, energy from the inductive load can be consumed using an avalanche region of the parasitic diode. The MOSFET therefore has relatively large avalanche energy resistance.
The avalanche energy resistance is an index of disruption endurance of a device, and is defined as maximum energy that can be consumed without causing disruption of the device when energy accumulated in the inductive load is consumed by the device.
On the other hand, a field effect transistor device of a compound semiconductor, such as a GaN-HEMT and a GaAs-HEMT, generally does not have a P region and therefore does not have a parasitic diode structure. Hence, energy from the inductive load cannot be consumed within the element and the energy exceeds a gate-drain withstand voltage (BVgd) and a source-drain OFF withstand voltage (BVdsoff) and eventually gives rise to an element disruption. Accordingly, it is general to use the HEMT with a protection element in a system of an inductive load having self-inductance L, such as an inverter.
FIG. 10A and FIG. 10B are views showing examples of a protection element connection. FIG. 10A shows an example where a diode is connected between the source and the drain. FIG. 10B shows an example where a diode is connected between the gate and the drain and between the gate and the source.
The connection configuration shown in FIG. 10A is described, for example, in JP-A-2009-164158. This connection configuration, however, has a drawback that the protection element occupies a large area because the diode requires a current capacity equal to a rated current of the MOSFET.
Also, the connection configuration shown in FIG. 10B has not been proposed for a GaN-HEMT. This connection configuration is, however, equivalent to a protection circuit of an IGBT element. A mechanism of this connection configuration is as follows. That is, when a gate-drain voltage rises, a zener diode between the gate and the drain starts to operate and the diode between the gate and the source starts to operate at the same time. Hence, a gate voltage is lifted up and the channel is opened, so that avalanche energy is released.
This connection configuration is of the mechanism by which when a drain voltage is increased by energy in the inductive load connected to the IGBT element, the channel is opened by modifying the drain voltage and transmitting the modified drain voltage to the gate voltage, so that avalanche energy is released. This connection configuration therefore has an advantage that a large protection element is not required.
Such being the case, a diode may be provided to a GaN-HEMT as a protection element as with an IGBT element. However, because the protection element of an IGBT element is formed of an Si diode, it is natural to form, for example, polysilicon to be formed into a diode on a GaN layer in the same manner. Because a GaN layer is semi-insulating, a polysilicon layer can be directly formed on the GaN layer. However, silicon goes into the GaN layer and becomes a dopant. In order to avoid this inconvenience, a polysilicon layer is formed directly on the GaN layer via an insulating layer. In this case, a parasitic capacity is undesirably formed by the GaN layer, the insulating layer, and the polysilicon layer.
As has been described above, it is difficult to provide a diode structure to a GaN-HEMT and a protection diode structure suitable to a GaN-HEMT has not been proposed to date.